As a sealing material, there is used a composite powder material comprising a glass powder and a refractory filler powder. This sealing material is excellent in chemical durability and heat resistance and suitable for securing airtightness as compared to a resin-based adhesive.
As the glass powder used for the sealing material, PbO—B2O3-based glass has been used (see, for example, Patent Literature 1). However, from an environmental view, a demand for removal of PbO from a glass composition has led to development of Bi2O3—B2O3-based glass. In addition, Patent Literature 2 or the like discloses that the Bi2O3—B2O3-based glass has a low melting point and has chemical durability comparable to that of the PbO—B2O3-based glass.
Meanwhile, the refractory filler powder used for the sealing material is added in order to achieve a decrease in thermal expansion coefficient and an improvement in mechanical strength, and as a material for the powder, lead titanate or the like having low expansion has been used. However, as with the glass powder, the refractory filler powder is demanded to have a composition from which PbO has been removed. Accordingly, as the refractory filler powder, there has been considered use of willemite, cordierite, tin dioxide, β-eucryptite, mullite, silica, β-quartz solid solution, aluminum titanate, zircon, and the like. Of those, willemite has low expansion and satisfactory compatibility with the Bi2O3—B2O3-based glass (hardly devitrifies the Bi2O3—B2O3-based glass at the time of sealing), and hence has attracted attention (see Patent Literature 3 and Non Patent Literature 1).